This proposal is for the examination of those factors which determine cell number in the adipose tissue of man and animals. Specifically, the time course of development of obesity in subsutaneous, epididymal and retroperitoneal fat of the Zucker obese rat and Bar Harbor obese hyperglycemic mouse will be studied to compare these strains which have hyperplastic adipose tissue, with other genetic strains showing only cell hypwereophy. The effect of sex hormones on control of adipocyte number will also be evaluated since only the female Bar Harbor hyperglycemic mouse shows marked hyperplasia. It is now clear from studies of tritiated thymidine incorporation into adipocyte DNA, that the replication of precursor adipocytes occurs in the first three to four weeks of life in the rat. Thereafter, the growth of the adipose depot is the result of filling of pre-formed cells. The nutritional, hormonal and genetic influences on the rate and time course of production of pre-adipocytes are the major variables under study.